When doing a LUMA blit, render to an intermediate texture. Instead of rendering the swizzled result to the destination texture, render to another intermediate texture and then CopyTexImage it into the destination texture. Rendering to arbitrary mip levels of the destination texture is not always supported but CopyTexImage-ing is. BUG=angleproject:1113 Change-Id: I352a1fcc769ce92268744d3e7a6265dd2bfddb37 Reviewed-on: https://chromium-review.googlesource.com/310570 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>